Preventing corrosion of ferrous metals



Patented Sept. 17,

UNET

Herman A. Beekhuis, Jr., Petersburg, Va and William De Forest Macomber,Clay, N. Y., as-

signers,

by mesne assignments, to

The Solvay Process Company, New York, N. Y., a corporation of New YorkNo Drawing. Application December 19, 1936, Serial No. 116,710

5 Claims.

I'he present invention relates to methods of reducing corrosion offerrous metals by ammonium nitrate solutions containing free ammonia.

In recent processes for the ammoniation of superphosphates, use is madeof solutions containing large percentages of ammonium nitrate and alsofree ammonia. The provision of meth-- ads which reduce or prevent thecorrosion of apparatus. in which these solutions are handled constitutesa major problem in the commercial use of these solutions, since, whenamonium nitrate-ammonia solutions of the concentrations used are shippedin tank cars of the usual con- ,8 struction, rapid corrosion occurs, andthe tank soon is not usable. Also corrosion occurs in apparatusconstructed of ferrous metals in which the solutions are handled.

It is an object of this invention to provide a method for reducing therate of corrosion of ferrous metals by corrosive solutions of ammoniumnitrate containing free ammonia, so that it is commercially practicableto employ apparatus of these metals for the storage and transportation35 of such solutions.

It is a further object of this invention to provide solutions ofammonium nitrate containing free ammonia which but slowly corrodeferrous metals.

)0 According to the present invention, the apparatus which may be astorage tank, tank car, conveying pipe, measuring tank or the like, .isconstructed of the ferrous metals commonly used for this purpose, suchas cast iron, wrought iron and low carbon steels, for example structuralsteel, which are subject to severe corrosion by ammoniacal solutions ofammonium nitrate.

We have found that particularly good results are obtained in minimizingthe corrosive effect on such ferrous metals of ammonium nitrate-ammoniasolutions by incorporating in the solution a compound of the groupconsisting of the thiocyanates and thiourea. These materials arecompounds containing divalent sulfur linked to an atom of carbon withthe remaining valences oi the carbon atom being satisfied by nitrogen.Thus, the thiocyanates, also called sulfocyanates and sulfocyanides,have the formula XS- CEN;

and thiourea has-the formula When a thiocyanate is the materialemployed, we prefer alkali metal (including ammonium) thiocyanates.However, we may employ alternatively other inorganic thiocyanates, whichincorporate the thiocyanate ion in the solution. For example, ferricthiocyanate may be added to the solution although the cation of thismaterial is relatively insoluble. By reaction with the solution, aferric compound is precipitated, but the thiocyanate ion is therebyincorporated in the solution. By the addition of a compound containingdivalent sulfur linked to an atom of carbon and the remaining valencesof the carbon atom being satisfied by nitrogen the rate of corrosion offerrous metals by ammoniacal ammonium nitrate solutions is reduced to'avery low rate.

The quantity of the thiocyanate or thiourea which may be added to reducethe rate of corrosion may vary from a very small percentage upward. Itis preferred, however, that the solution contains 0.01% to 2% of thesematerials. More than 2% may be present although the presence in thesolution of a high proportion of the thiocyanate or thiourea is notnecessary to accomplish the purposes of the invention. For example,solutions made up of 60 parts ammonium nitrate, parts ammonia and 20parts water to which 0.05% ammonium thiocyanate or thiourea was added,corroded an assembly of low carbon steel and cast iron at only a smallcordingly, it is desirable to incorporate in the solution an amount ofthe thiocyanate or thiourea which is a minor proportion of the amount ofammonium nitrate in the solution.

This invention has specific application to aqueous solutions of ammoniumnitrate containing free ammonia in which the amonium nitrate constitutesabout 50% or more of the solution (for example, 40% to 80%) and in whichthe ammonium nitrate is dissolved in 5% or stronger aqua ammonia,preferably in an aqua ammonia of about 25% or greater strength, due tothe relatively high rates of corrosion exhibited by these solutions.

This invention also has specific application to contacting, attemperatures below about 50 0., ferrous metals with solutions containingammonium nitrate and free ammonia in which the thiocyanate or thioureahas been incorporated.

In order that this invention may be more clearly understood, thefollowing examples typical of preferred methods of procedure are givenby way of illustration:

Example I .To a solution containing 60 parts ammonium nitrate, 20 partsammonia and 20 parts water, 0.1% of sodium thiocyanate is added. Thesolution is then introduced into a container made of low carbon steel orcast iron and stored in such a container at room temperature.

Very little corrosion of the carbon steel or cast iron occurs duringlong periods of contact with the solution. For example, the rate ofcorrosion of a specimen of low carbon steel in the above solutionscontaining 0.1% sodium thiocyanate was less than 2% of the rate ofcorrosion of the same steel in the ammonium nitrate-ammoniawatersolution without thiocyanate being added. With thiocyanate added therate of corrosion of cast iron by the solution was but 2% of the ratewithout thiocyanate.

In place of the sodium thiocyanate used in this example, another alkalimetal thiocyanate (including ammonium thiocyanate) may be added to theammonium nitrate-ammonia-water solution.

Example lI.To a solution containing 60 parts ammonium nitrate, 20 partsammonia and 20 parts water, 1% of thiourea is added. The solution isthen introduced into apparatus constructed of cast iron and low carbonsteel at room temperature.

Very little corrosion of the cast iron and steel by the solution occurs.It has been found the rates of corrosion of the cast iron and steel arereduced to but 3% and 1%, respectively, of the rates of corrosion ofthese materials by a solution of the same composition but containing nothiourea.

The following theory is ofiered as a possible explanation of theeffectiveness of the above processes for inhibiting corrosion of ferrousmetals by ammoniacal ammonium nitrate solutions. It appears from ourobservations of the action of numerous salt solutions on ferrous metalthat the chemical reaction which causes rapid corrosion of ferrousmetals by ammoniacal ammonium nitrate solutions is peculiar to nitratesolutions and ferrous metals. Further, in the ease oi the nitratesolutions, rapid corrosion of the metal as a result of this reactionoccurs only with the ammoniacal solutions. Thus, in the case of acid andneutral solutions of ammonium nitrate, hydrogen is evolved by thecorrosion of ferrous metal by the solutions. In the case of corrosion byammoniacal ammonium nitrate solutions, in-

stead of detecting the evolution of hydrogen, we

have observed that ammonia is formed by reactions taking place duringcorrosion of the metal compound containing divalent sulfur linkedtocarbon, of which class of compounds the thiocyanates and thioureadisclosed and claimed herein are examples.

This application is in part a continuation of our copending applicationSerial No. 726,734, filed. May 21, 193%.

We claim:

1. The process of reducing corrosion of ferrous metals by a solution ofammonium nitrate con taining free ammonia which corrodes ierrous metals,which comprises dissolving in said som 1 tion a thiocyanate in amountsufficient to ma terially decrease the rate of corrosion of a fer rousmetal by said solution, and contacting said.

solution with said ferrous metal.

2. The process of reducing corrosion oi ierrous metals by a solution ofammonium nitrate con taining free ammonia which. corrodes said lerrousmetals, which comprises dissolving an alkali metal thlocyanate in saidsolution in amount sufilcient to materially decrease the rate of cor-=-rosion of a ferrous metal by said solution, and contacting said solutionwithsaid ferrous metal.

3. The process-of reducing corrosion of ferrous metals by a solutioncontaining about 56% or more ammonium nitrate dissolved in about 25% orstronger aqua ammonia, which comprises dissolving 0.01% to 2% oi analkali. metal titlecyanate in said solution, and contacting saidsolution with said ferrous metal.

4. As a new composition of matter suitable for storage, transportationand use in contact with ferrous metals, 9. soTution containing freeammonia, ammonium nitrate and a thiocyanate in amount suflicient tomaterially decrease the rate of corrosion of a ferrous metal by saidsolution.

5. As a new composition of matter suitable for storage, transportationand use in contact with ferrous metals a solution containing about 50%or more ammonium nitrate in solution in about 25% or stronger aquaammonia, in t'v'hich is dlssolved 0.01% to 2% of a tl'iiocyanate.

HERMAN A. BEEKHUIS, 5a. WILLIAM DE FOREST MACOmEE.

